Intense efforts are being exerted by industry to extract thermal energy from the plentiful supplies of coal in the country. At present, the fluidized bed appears attractive.
There are two general types of fluidized beds in which to burn crushed coal. First, there is the atmospheric bubbling fluidized bed up through which combustion air flows at 6-12 ft./sec. Secondly, there is the turbulent, or circulating, fluidized bed which utilizes combustion air velocities of 15-35 ft./sec. Each of these beds has unique advantages and limitations.
The principle of a turbulent or circulating fluidized bed is well established and its operation has been previously demonstrated in alumina calcining plants and in other type chemical process plants. Development is going forward rapidly on this type of bed to adapt it for the combustion of coal for commercial application. There are high hopes that this type of bed will burn coal with high efficiency and have low emissions of SO.sub.2. This system is peculiarly adapted to the combustion of small size coal (down to 500 mesh). The general advantage of this type fluidized bed is the achievement of high combustion efficiencies and the much lower limestone requirements for achieving high SO.sub.2 removal. The disadvantage, at least in commercial size plants, is the very large size of separation equipment (cyclones) and furnace. These large sizes mean, of course, a higher cost of equipment, of structural steel, plant building size, and high air fan power.
Turning back to the atmospheric bubbling type of fluidized bed, its development thus far shows it to have certain advantages and disadvantages relative to the turbulent, or circulating, fluidized bed. Utilizing the lower velocity of combustion air (6-12 ft./sec.) the utilization of the limestone necessary to absorb the sulfur compounds within the coal has not been as efficient as hoped. The necessary weight ratio of calcium-to-sulfur hovers around the high value of 4:1 to reduce the SO.sub.2 content of the combustion gas discharged to the atmosphere in meeting present environmental standards. However, this bed has an advantage in that it has the potential for being built in large sizes (one process unit) thereby having the potential for lower capital cost.
The present problem faced is how to combine the two types of beds to optimize the efficiency of coal combustion and limestone utilization, while keeping the capital cost of the equipment within a reasonable range.
The present system of fluidized bed operation should be staged with these two types to gain overall efficiency in the extraction of energy from solid fuel with this mode of combustion. Dividing the combustion of the coal and capture of the sulfur compounds into a plurality of operational stages appears attractive. Different conditions of combustion and absorption of sulfur compounds in the separate beds can be maintained and adjusted independent of each other to the gain of overall efficiency.